For various applications utilizing rubber which require high strength and abrasion resistance, particularly applications such as tires and various industrial products, sulfur cured rubber is utilized which contains substantial amounts of reinforcing fillers. Carbon black is commonly used for such purpose and normally provides or enhances good physical properties for the sulfur cured rubber. Particulate silica is also often used for such purpose, particularly when the silica is used in conjunction with a coupling agent. In some cases, a combination of silica and carbon black is utilized for reinforcing fillers for various rubber products, including treads for tires. The use of such reinforcing fillers for elastomers, including sulfur curable elastomers, is well known to those having skill in such art.
It is important to appreciate that, conventionally, carbon black is a considerably more effective reinforcing filler for rubber products, and particularly for rubber tire treads than silica if the silica is used without a coupling agent, or silica coupler as it may be sometimes referred to herein.
Often coupling agents are composed of a silane which has at least one first constituent component, or moiety, (such as a silane portion) capable of reacting with the silica surface and, also, an additional moiety capable of interacting with the rubber, particularly a sulfur vulcanizable rubber which contains carbon-to-carbon double bonds, or unsaturation. Usually the additional moiety is sulfur in a form of a polysulfide and particularly a polysulfide bridge between said first moieties. In this manner, then the coupler acts as a connecting bridge between the silica and the rubber and thereby enhances the rubber reinforcement aspect of the silica.
The rubber-reactive group component, namely the said additional moiety, of such coupler may be, for example, one or more of groups such as mercapto, amino, vinyl, epoxy, and sulfur groups, preferably a sulfur or mercapto moiety and more preferably sulfur in a form of a polysulfide as a polysulfide bridge between at least two of said first moieties.
Numerous of such coupling agents are taught for use in combining silica and rubber such as, for example, silane coupling agents containing a polysulfide component, or structure such as, for example, trialkoxyorganosilane polysulfides such as, for example, bis-(3-trialkoxysilylorgano) polysulfides, containing from about 2 to about 8 sulfur atoms in a polysulfide bridge such as, for example, bis-(3-triethoxysilylpropyl)tetrasulfide and/or trisulfide.
Various U.S. patents relating to silicas and silica reinforced tire treads include, for example, U.S. Pat. Nos. 3,451,458; 3,664,403; 3,768,537; 3,884,285; 3,938,574; 4,482,663; 4,590,052 and 5,089,554.
In one aspect, and in contrast to the aforesaid rubber reactive silica coupling agents, other silica-reactive materials might be utilized to interact with the silica which have only one constituent component, or moiety, and which is capable of reacting with the silica surface. Such materials are not readily reactive with the rubber so that they do not readily promote interaction between the silica and the rubber. Therefore, by itself, such material is not considered herein to enable the silica to satisfactorily reinforce the rubber.
The term "phr" if used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber, or elastomer".
In the description of this invention, the terms "rubber" and "elastomer" if used herein, may be used interchangeably, unless otherwise prescribed. The terms "rubber composition", "compounded rubber" and "rubber compound", if used herein, are used interchangeably to refer to "rubber which has been blended or mixed with various ingredients and materials" and such terms are well known to those having skill in the rubber mixing or rubber compounding art.